One approach to locating objects on the surface of the earth is the Global Position System (GPS). The GPS includes a constellation of satellites broadcasting predetermined transponder signals. Terrestrial-based GPS receivers can accurately compute their locations based on these transponder signals. Although GPS can provide accurate positioning information, its limitations make it impracticable in certain applications. For instance, using GPS in a mobile communications system to locate user terminals (UTs) has several disadvantages. First, GPS receivers are expensive. Requiring each UT in a communications system to include a GPS receiver would dramatically increase costs. Second, on cold-start power up, it often takes a GPS receiver several minutes to acquire its position. This lengthy acquisition time is impracticable in certain satellite mobile communications systems in which UT position determination should be performed at the beginning of a phone call without noticeable delay to the user.
Another approach, Qualcomm's Automatic Satellite position reporting used in its OmniTRACS product, determines the position of a user terminal on the surface of the earth using two geosynchronous satellites with the terminal measuring the absolute delay to one satellite, as well as the differential value between the delays to the two satellites. The absolute delay to the second satellite is obtained by adding the differential delay to the absolute delay to the first satellite. The position of the terminal can then be calculated from the two absolute delays and the known positions of the two satellites (with the assumption that the terminal is on the earth surface). There are two potential problems that make this approach inappropriate for certain satellite communications systems: 1) This approach does not provide high accuracy for low latitude regions (around the equator) due to geometric dilution of precision (GDOP) as explained later in this disclosure. This problem prevents the approach from being applicable to a system that is to provide global position determination service. 2) This approach requires that the differential delay measurement be made at the same instance of time as the absolute measurement is made or that the two satellites be stationary, to derive a meaningful second absolute delay. This requirement adds complexity to the terminal design in a system that employs fast moving satellites (with low or medium orbits). The requirement also lengthens the call setup time to a system in which position of a terminal must be acquired before a call.
A third approach to determining UT position relies on measured signal propagation delay and Doppler shift from the radio transmission link between a satellite and the UT. Typically, the propagation delay and Doppler shift are derived from a radio-frequency (RF) carrier transmitted between the UT and a moving transceiver, for example, an RF transceiver included on a moving airplane or satellite. The Doppler shift is a well known physical phenomenon and represents the observed change in frequency of the propagated RF wave that occurs due to the relative motion between the UT and the transceiver. The measured signal propagation delay is the amount of time required for an electromagnetic signal to travel between the UT and the moving transceiver. From this delay, it is easy to calculate the distance separating the UT and transceiver by multiplying the delay by the speed at which the electromagnetic signal travels, which is generally at or near the speed of light.
In the above one-satellite UT position determination method, a ground station can measure the propagation delay and Doppler of a communication link between the UT and a satellite. The ground station then calculates the UT position based on the measurements and the predetermined satellite position at the time of the measurement. As shown in FIG. 1, the measured values of propagation delay and Doppler can be conceptually mapped onto the surface of the earth, revealing UT locations corresponding to the measurements. Since propagation delay and Doppler measurements have inherent errors, each measurement is represented as a band along the corresponding contour. The intersecting areas of the two bands, which are shown cross-hatched, indicate regions where the UT can be located. The measurements by themselves typically give two possible areas where the UT can be located. The specific area actually containing the UT can be identified by additional information, such as the identification of the satellite spot beam servicing the UT.
Locating a UT using propagation delay and Doppler measurements from a single satellite communication link suffers from the intrinsic property of geometric dilution of precision (GDOP), meaning that if the UT is within certain areas on the surface of the earth relative to the satellite, the two measurements do not provide accurate positioning information.
This phenomenon is illustrated in FIG. 2 for a one-satellite method, which is a conceptual diagram showing a typical set of propagation delay contours and a typical set of Doppler shift contours on a portion of the surface of the earth visible to the satellite. The concentric circles represent propagation delays of constant values, and the quasi-hyperbolic curves represent constant Doppler shift values. The center of the concentric circles represents the sub-satellite point, which is the point on the earth directly below the orbiting satellite. The straight line represents the satellite ground track, which indicates the instantaneous direction of travel of the satellite relative to the surface of the earth.
The amount of error in a UT position estimated using a one-satellite method depends largely upon the location of the UT relative to the satellite. The GDOP is most pronounced on and near the ground track, where the estimated area in which the UT can reside elongates, significantly reducing the accuracy of the UT position estimate. Thus, for UTs near the ground track, the one-satellite locating method does not produce good estimates of UT positions. However, at locations away from the ground track, the areas defined by intersecting Doppler and delay bands shrink, indicating a more accurate estimate.
Accordingly, there is a need for a method and system that improves the accuracy of UT position determination by reducing the effect of GDOP.